Field of the Invention
The invention relates to a method and a device for eliminating rhythmic register errors in sheet-fed rotary printing machines.
In sheet-fed rotary printing machines with a transport drum of double the conventional printing-unit cylinder size, in particular an impression cylinder of double size, and, as a consequence thereof, having two gripper arrangements, the problem arises that an error in the unwinding of successive sheets in relation to the rubber blanket cylinder, and a consequent print offset occur due to manufacturing or assembly tolerances in the components of the impression cylinder. Two successive sheets, which are therefore transported alternately by different gripper arrangements, consequently have a circumferential offset in relation to one another. The result of this is that the sheets receive their print at a slightly offset location in a two-by-two rhythm. During a single run of a sheet through the printing machine, this small offset influences only the length of the print-free sheet edge, which is virtually of no significance.
A problem arises only when a print order is to have a second print run, for example when a four-color print is to be produced by a two-color machine.
In this case, then, in order to avoid the offset between the print image of the first run and that of the second run, it will be necessary for the impression cylinder gripper arrangement which grasped the sheet during the first print run also to transport the same sheet during the second print run.
It has been possible heretofore to satisfy this requirement only by executing a specimen proof print at the start of the second print run. The print result indicated whether the sequence of the sheets supplied was correct. If this was not so, it was possible for the pressman to correct the sequence by removing a sheet from the sheet pile or feed table. Moreover, after damaged sheets had been removed, this operation possibly had to be repeated during the processing of the order (for example, by stopping at the feeder due to multiple and oblique sheets, respectively, or overshooting), because the correct relationship was lost again due to the random withdrawal or extraction of an odd number of sheets. These measures, however, require a considerable amount of time, and an appreciable quantity of spoilage occurs.
The published German Patent Document DE 196 18 029 A1 can solve the problem described above by gripper arrangements on a sheet transfer cylinder, the gripper arrangements having adjustable closing times.